Fixing means for the base of a radioactive material transport and/or storage container

ABSTRACT

A container for transport and storage of highly radioactive material, the container being made of thick metal and comprising a tube having an internal wall and a base having a lateral wall, the base being non-removably sealed to one end of the tube, and the internal wall and lateral wall forming a right cylinder with circular cross-section in contact with each other. The base is fixed to the container by shrink fitting the lateral wall with a portion of the internal wall in contact with the lateral wall, the base being disposed such that the lateral wall is entirely inside the tube with a portion of the internal wall comprising an opposed shoulder which cooperates with the corresponding shoulder in the lateral wall. A first continuous seam weld is provided on an external surface of the base in contact with the tube and a second continuous weld seem is provided on an internal surface of the base in contact with the tube.

FIELD OF THE INVENTION

The invention concerns a fixing means for the base of a transport and/orstorage container for highly radioactive material, in particular forirradiated nuclear fuel assemblies or their highly active reprocessingwaste.

DESCRIPTION OF RELATED ART

Containers for highly radioactive material generally comprise a thickshielded chamber which confines the material, stops gamma radiation andis mechanically strong, even under accidental severe Conditions.

They are generally cylindrical with a cross section which is eithercompletely circular or is provided with flats. They comprise a tubewhich is closed at one end by a base which is permanently fixed thereto.The other end constitutes the main opening which is closed by aremovable plug or cover which is often complex, for filling and emptyingthe contents. These closures must remain sealed (sometimes even tohelium) when under accidental severe conditions, in particular afterregulation drop tests, for example a drop of 9 m along a tubegeneratrix, on a corner of the container, on the base or on the cover,including penetration drops.

The main frame of the tube and closures can be formed of a thick metalwall of high mechanical strength, for example a steel, which can beseveral tens of centimeters thick; thus containers for transport and/orstorage of nuclear fuel assemblies or vitrified waste, the steel tubecan be more than 25 or 30 cm thick, similarly the base and the maincover, and the unladen weight of the container assembly can be up to 120t; its laden weight can be 150 t or more. The radiological shieldingprovided by the frame can be supplemented by layers of appropriatematerials outside or inside the container, on the tube or on the endclosures.

A known container type is illustrated in FIG. 1, which shows:

thick tube (1), for example of steel, covered with further layers (2),(3) of radiation absorbing material. The main closure system comprisestwo covers, a primary cover (4) and a secondary protective cover (5),both removable. Particular fixing or monitoring apparatus are not shown;

a non removable base (6) which in this case is fixed to tube (1) by aweld (7) through the entire thickness of the tube;

handling lugs (8) which are generally welded across weld (7).

A weld of type (7) is long and difficult to make because of the greatthickness of the steel which must be welded; great care is necessary andmany checks must be carried out during manufacture because the weldalone holds the base in position and provides the drop strength as wellas sealing the container.

This welding problem has been simplified by providing a further closuremeans as illustrated in FIG. 2. Base (6) has an external diameter whichis equal to that of tube (1); it includes a Shoulder (9) at theperiphery of the internal surface (19), which cooperates with theinternal diameter of tube (1) and allows base (6) to be cold assembledwith a light friction fit, fitted partially inside the tube and abuttingthe cross sectional surface of end (10) of tube (1). A peripheral weldseam, which is simpler to make and check than the weld described above,holds base (6) in place. It can be completed by a second weld seam (18)on the inner face.

In this type of assembly, here again a weld ensures that base (6) isheld in place and seals the tube; the welds are extremely stressed ifthe container is accidentally dropped along a tube generatrix or at anangle to the base. As before, then, they must be made with extreme careand closely checked. They constitute a weak point there is a risk ofrupture which cannot guarantee a complete seal in the event of a severeshock. European patent EP-A-0 061 400 describes a closure means for acontainer for radioactive material in which the seal is provided solelyby Shrink fitting the (removable) cover into a tube (p.2, 1.37-38 top.3, 1.30) which is brought about by an absence of faults in thecontacting surfaces (p.4, 1.5-6). It also describes a boss (7) locatedOn the tube on either side of the sealing surfaces, which is not inpermanent contact with the removable cover and whose size is linked tothe expansion of the tube. This thus creates an obstacle which cannotresist axial displacement of the cover (p.3, 1.31-32, 35-36, 39). Frenchpatent FR-A-2 092 502 describes a vacuum seal which is obtained byshrink fitting, one of the shrink fitted pieces being provided with anedge which deforms on contraction. European patent EP-A 0 101 362describes a sealed closure for a removable cover produced by shrinkfitting two conical portions, for a container for the transport ofradioactive material; the contacting surfaces must be made with care(p.2, 1.9-10); the closure comprises an axial abutment and the coverprojects beyond the tube.

These various assemblies cannot guarantee a seal in the event of asevere shock.

SUMMARY OF THE INVENTION

We have sought to overcome these problems and develop a means forassembling a base which is more secure and which avoids weak points withtheir associated rupture risks or affect the seal of the container inthe event of the container being dropped, in particular horizontal oroblique drops, which are usually more severe, but also vertical drops,on the base or on the cover, including penetration drops.

We have also developed a simple assembly which is economical tomanufacture.

The invention therefore concerns a means for fixing the base of ashielded container for transport and/or storage of highly radioactivematerial, comprising a tube and a non removable base of thick metal, forexample steel, the tube and base having respectively, at least over acertain height, an internal wall and a lateral wall forming a rightcylinder with a circular cross section in contact with each other, thebase being held in place by shrink fitting its lateral wall with theportion of the internal wall of the tube in contact therewith,characterised in that the base is located at least partially inside thetube, in that said portion of the internal wall of the tube comprises ashoulder which cooperates with a corresponding opposed shoulder in thelateral wall of the base and in that the base is connected to the tubeby a continuous weld seam on its external surface and by a further weldseam on its internal surface.

The base-tube closure is permanent.

The means of the invention ensures that the base will not displacerelative to the tube, due to the shrink fitting in the event of ahorizontal drop, due to the opposed shoulder in the event of a verticaldrop on the base, and due to the combination of these two elements inthe event of an oblique drop on the base. Thus, the weld seams whichprovide a perfect seal to helium only suffer weak stresses, for exampledue to the contents of the container rebounding in the event of avertical drop on the cover. The welds can thus be smaller.

In general, a first weld seam connects the peripheral edge of theexternal surface of the base to the internal edge of the end face of thetube or the internal surface of the tube, the recessed base thusproviding a volume which can usefully accommodate, for example, anadditional incompressible neutron shield. Similarly, a second weld seamconnects the peripheral edge of the inner surface of the base to theinner wall of the tube. These weld seams seal the container, inparticular to confine radioactive material in the container cavity or toavoid contamination when immersed in a cooling pond. They are not highlystressed mechanically in the event of a drop, even in the event of avertical or an oblique drop on a lower edge of the container, or eventin the event of the load rebounding against the base in the event of adrop on the cover.

Thus the means of the invention not only significantly reduces thevolume (up to -95%) and size of the welds, but also the specificationsfor these welds are less demanding. In particular, checks are simplifiedcompared with those carried out of prior art welds, which latter play amajor mechanical role in holding the base on the tube. This facilitatesmanufacture and provides cost advantages.

Shrink fitting ensures the absence base and the tube, by preventing anyrelative movement between these two pieces during a drop, thusmaintaining the integrity of the welds.

To obtain this result, the facing surfaces during shrink fitting do nothave to be as carefully prepared as would be necessary if the shrinkfitting had to provide the seal for the container.

It should be noted that, in the means of the invention, the weld seamscan also prevent the onset of corrosion at the base--tube interface,which corrosion can occur when the container is immersed in a coolingpond or as a result of condensation from the atmosphere and which woulddamage the base--tube joint; they can also prevent contamination fromentering this interface.

The shoulder in the inner surface of the tube is in intimate contactwith the corresponding opposed shoulder in the lateral wall of the base;it reduces stresses in the welds, primarily in the event of a verticalpenetration drop in the centre of the base. It also ensures exactlocation of the base with respect to the tube.

Shrink fitting is generally carried out on the small and the largediameter to produce double shrink fit. It can also be effected solely onthe small diameter (towards the container cavity); in both cases, theheight of the opposed shoulder of the base resting on the shoulder ofthe tube can be adjusted so that its transverse strength is sufficient.Advantageously, the forged base is located substantially inside thetube, the external surface of said forged base or that of the optionalcomplementary neutron shielding being flush with the end of the tube;this disposition distributes the shock over the base and tube in theevent of a vertical drop.

Shrink fitting is effected by ensuring that the lateral wall of the basehas a diameter which is slightly greater than that of the correspondinginternal wall of the tube. The base is fitted into the tube after thetwo components have been heated to temperatures which are sufficientlydifferent to provide a suitable temporary allowance for assembly.

After fitting, the external surface of the base advantageously does notgo beyond the plane containing the end face of the tube, the base thusbeing located entirely within the tube. After cooling, the shrinkfitting force develops over the whole of the lateral wall of the base,or over only a portion of its thickness, and is sufficient to hold it inplace.

The shrink fitting is the stronger the larger the difference, when cold,between the diameter of the lateral wall of the base and that of thecorresponding internal wall of the tube recommended, however, that thisdifference is kept below a limiting value above which the tensionalstrains in the tube and/or compressional strains in the base would gobeyond the accepted threshold for the material used.

With the means of the invention, the shrink fitting force can beregulated by adjusting the value of the excess of the external diameterof the base when cold over the internal diameter of the correspondinginternal wall of the tube, also when cold.

By way of example, when the base and tube are of steel and for adifference between their diameters of between 0-5 and 1 per thousand(which would necessitate a temperature difference of the order of 200Kduring assembly), the shrink fitting force at the interface can be ofthe order of 100 MPa, which is an acceptable value for most types ofsteel.

When the cavity of the container has a non circular cross section, ingeneral the internal wall of the tube which contacts the base ismachined to produce a circular cross section which cooperates with thecircular lateral wall of the base during shrank fitting

A wide variety of thick metals can be selected to form the tube and thebase. The choice can be guided by mechanical properties, corrosionresistance, protection against radiation. etc. . . If required,different metals could be used for the base and tube- Preferably, themetal is selected from steels (optionally stainless) copper and itsalloys, for example bronzes, etc. . .

Advantageously, the tube comprises handling lugs fixed on the externalwall close to the base and the cover. With the fixing mode of theinvention, the lugs near the base are fixed directly to the tube, forexample by welding, and the weld does not interfere with other welds,unlike those of the prior art (see, for example, FIGS. 1 and 2). Weldswhich cross over One another generally run the risk of mutualembrittlement, and thus the absence of weld interference is anadditional advantage of the invention.

The means of the invention, comprising a base which is held in place byshrink fitting, is particularly suitable for fixing the base ofcontainers for highly radioactive material with very thick walls (baseand tube) of metal, for example steel, typically 10 cm to 50 cm thick(generally 20 to 30 cm thick) and weighing more that 10 t (generally 70to 150 t),

Cooperation of the means employed in the invention also means thatcontainers can be produced which are simple and thus economical to make,which satisfy specifications for transport and/or storage containers,including those for liquids, and in particular satisfy the requirementsimposed by international regulations governing drop conditions oraccidental severe shocks (including horizontal drops along a tubegeneratrix or obliquely at an angle to the tube near the base), while atthe same time reducing the checks required during manufacture. Thestresses occurring during a severe impact do not directly affect theweld seams of the base--tube interface.

Thus with the means of the invention, the forces which occur duringsevere shocks or drops do not damage the base--tube joint, thusimproving not only the seal security but also corrosion protection andcontamination protection of said base--tube interface. In addition, asimpler method of manufacture is employed which uses only knownmachining or welding techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a vertical cross-sectional of a prior art container;

FIG. 2 shows a cross-section of a second prior art container;

FIG. 3 shows a vertical cross-section of a first container according tothe invention;

FIG. 3a shows in detail a portion of the container of FIG. 3; and

FIG. 3b shows in detail a portion of a second container according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 shows a container with a non removable base in accordance withthe present invention. FIG. 3a shows an enlargement of the base--tubeinterface. FIG. 3b shows a detail of the base--tube interface in theparticular instance where the base is recessed within the base.

Reference numerals 1, 2, 3, 4, 5, 6, 8, 10, 18, 19 have the samemeanings as in FIGS. 1 and 2 of the prior art. Base (6) is locatedinside tube (1), of steel or other strong metal, and has a peripherallateral wall (12) which is cylindrical with a circular cross section andis enclosed over its entire height by an identical cylindrical portion(13) machined in the internal wall (20) of tube (1). In the figure, theexternal surface (14) of base (6) does not extend beyond the planecontaining the end face (10) of tube (1). Base (6) is then held in placeby shrink fitting using the tube (1) itself.

The cavity (22) of the container can be of any shape (for example with apolygonal cross section) such that its internal wall (20) requires acountersink (21) (FIG. 3a) to hold the base in place while other cavity(22) shapes (for example a circular cross section) would not requirethis countersink (see FIG. 3, 18a).

The following can also be seen:

the external weld seem (17) connects the peripheral edge of the externalsurface (14) of base (6) to the tube;

the internal weld seam (18, 18a) which connects the peripheral edge ofthe internal surface (19) of base (6) to the tube; and at (18a), theinternal wall (20) of the cavity has no countersink (as describedabove).

Two cooperating shoulders (15) can also be seen, one machined in theinner wall of tube (1) and the other in the lateral (12) of base (6).

It can be seen from FIG. 3b that the external surface (14) of base (6)is not flush with the end face (10) of tube (1), but is within it: thisforms a circular disk which can, for example, be used to hold additionalincompressible neutron shielding (23). The sealing weld (17) thusconnects the external surface (14) of base (6) to the internal surfaceof tube (1).

It should be noted that it could be advantageous to provide an allowancefor assembly between base (6) and tube (1) in the vertical largerdiameter portion (arrow 13) from the shoulder and it is preferable thatthis portion is of reduced height with respect to the smaller diameterportion (arrow 12). This is preferably sufficient to hold the base (6)in place even if it forms no part of the secure shrink fitting.

It should also be noted that, as described above, the countersink (21)of the tube can be partially or completely absent over the internalsurface of the tube and may or may not coexist with shoulder (15). Itmay also replace shoulder (15).

What is claimed is:
 1. In a container for transport and storage ofhighly radioactive material, the container being made of thick metal andcomprising (i) a tube having an internal wall, and (ii) a base having alateral wall, said base being non-removably sealed to one end of thetube, and said internal wall and lateral wall forming a right cylinderwith a circular cross section in contact with each other,a means forfixing the base of the container, said means comprising:(a) a shrinkfitting of the lateral wall with a portion of the internal wall incontact therewith, the base being disposed such that the lateral wall isentirely inside the tube with said portion of the internal wallcomprising an opposed shoulder which cooperates with a correspondingshoulder in the lateral wall; (b) a first continuous weld seam on anexternal surface of the base in contact with the tube, and (c) a secondcontinuous weld seam on an internal surface of the base in contact withthe tube.
 2. A means according to claim 1, wherein the external surfaceis flush with an end face of the tube.
 3. A means according to claim 1,wherein the external surface is recessed in an end face of the tube. 4.A means according to claim 1, wherein the internal wall includes acountersink to hold the base in place.
 5. A means according to claim 1,wherein the metal is selected from the group consisting of steels,copper, copper alloys, and bronzes.
 6. A means according to claim 1,wherein handling lugs are fixed to a solid external wall of the tubenear the base.
 7. A container for transport and storage of highlyradioactive material comprising:a) a thick metal tube having first andsecond ends and an internal wall; b) a thick metal base permanentlysealing the first end of the tube, said base including an externalsurface, an internal surface and a lateral wall forming together withsaid internal wall a right cylinder of circular cross section in contactwith each other, said lateral wall being disposed entirely inside thetube; c) a shoulder portion on said internal wall at said first end; d)a corresponding shoulder portion on said lateral wall, constructed andarranged for engaging the shoulder portion on said internal wall; e) afirst weld seam between said external surface and the tube; and f) asecond weld seam between said internal surface and the tube, said tubeand said base being in shrink-fit relationship at said first end.
 8. Acontainer according to claim 7, wherein said external surface is flushwith said first end of the tube.
 9. A container according to claim 7,wherein said external surface is recessed in said first end of the tube.10. A container according to claim 7, which is helium tight.
 11. Amethod for permanently joining a thick metal tube having an internalwall and first and second ends to a base having a lateral wall andexternal and internal surfaces, to form a container for transport andstorage of highly radioactive materials, comprising the steps of:a)forming a shoulder on the internal wall at the first end of the tube; b)forming a corresponding shoulder on the lateral wall of the base,constructed and arranged for engaging the shoulder on the internal wall;c) shrink fitting the base and the tube with the corresponding shouldersengaged and with the lateral wall entirely inside the tube; d) forming afirst continuous weld seam between the external surface and the internalwall; and e) forming a second continuous weld seam between the internalsurface and the internal wall.